1. Field of the Invention
This invention relates to the preparation of polymeric fibers in general and to the formation of such fibers from solution in particular.
2. Prior Art
The formation of fibers from polymeric materials by spinning from a melt or from a highly viscous solution is old. These methods involve the initial formation of the fiber by a mechanical step such as extrusion through a spinnerette or drawing a fine continuous thread from the viscous melt. In the case of fiber formation from the molten material, the fibers are cooled and subsequently stretched and heat treated to develop desired mechanical properties. In fibers spun from highly concentrated solvent solutions, the solvent is removed by evaporation or extraction following the spinning step. They are then mechanically stretched and heat treated in the same manner as fibers formed from the melt.
In recent years, the formation of fibers by stirring very dilute solutions of certain polymers has been described in the scientific literature, Pennings, A. J., Vander Mack, et al. (Polymere, 99 (1969)). This procedure results in the formation of linear fibers attached to the stirrer. The fibers are non-uniformly distributed around the stirrer and are spirally arranged. The formation of similar fibers from stirred solutions has also been reported by A. Keller (Physics Today, May 1970, page 42). In addition, a crystalline material of fibrous shape having what is described as a "shish kebab" structure has been formed by irradiating a dilute solution of polyethylene in p-xylene with ultrasonics at 0.1 4 mW/cm from 85 to 190 Kilo HZ at a temperature of between 82.degree. to 88.degree. C. The concentration of polyethylene in the solvent ranged from 0.05 to 0.5% by weight and the fibers formed were sparse and very short (Blackadder and Schleinetz, nature 200, 788, 1963).
There is disclosed in U.S. Pat. No. 4,127,624 (Ser. No. 792,838, a division of Ser No. 616,747, abandoned) a process for generating unique masses of fibers from solutions of certain polymers by agitating the solutions at sonic frequencies. The fiber masses so formed consist of coherent, interconnected three dimensionally arrayed networks of very fine fibers. The method of forming fiber masses by sonic agitation of a cooling polymer solution was found to be useful with linear organic polymers, having a regularly repeated chain structure and a degree of crystallinity as determined by x-ray diffraction. The class of polymers described as polyalkenes are particularly amenable to fiber formation by the sonic agitation procedure. In general, isotactic polypropylene most readily formed fibers under these conditions and mixtures of isotactic polypropylene and other polyalkenes form fiber masses more readily than the polyalkenes alone. Non-crystalline polymers do not readily form fibers from solutions by the process disclosed in U.S. Pat. No. 4,127,624.
The ability to form dense fibrous masses from solution is highly desirable, particularly if the solute is a curable polymer or a curable polymer may be infiltrated into the mass after it is formed. This capability would facilitate the fabrication of superior solid encapsulants for complex geometry electrical devices.
Applicants know of no prior art processes for preparing fibers, fiber masses and fiber reinforced composites from non-cystalline polymers wherein the fibers are formed from solution in a manner which facilitates the fabrication of self-reinforced insulating elements within intricate electrical components such as complex shaped transformers magnetic devices and/or capacitors.